This invention relates to seismic exploration of earth formations and, more particularly, to methods and apparatus for acquisition and processing of marine seismic data from earth formations.
In seismic data acquisition and processing, it has been common practice to form arrays comprised of weighted sums of spatially distributed groups of seismic detectors for purposes of improving signal to noise ratios and the like well known in the art.
A general discussion of the theory and practice of such arrays may be found in articles entitled "Waveguides, Arrays, and Filters", Geophysics, Vol. 31, No. 3, pp. 501-505, by C. S. Clay, and "Characteristics of Seismic Streamers", Geophysics, Vol. 35, No. 6, pp. 1054-1075, by J. W. Bedenbender et al., which are incorporated herein by reference.
Two general approaches have been taken regarding the problem of forming these arrays. With reference to FIG. 1A, the first approach has been to simply record all of the analog detector group signals G.sub.1. . . G.sub.n on tape 6 in the field, convert the data to digital form by analog-to-digital converter 7, and then feed the result as digital tape 8 into a general purpose processing computer 9 for purposes of deriving the desired arrays. A variation on this technique, as shown in FIG. 1B, is to provide for some form of digitization in the field, such as analog-to-digital converter 10, the output of which is recorded as digital tape 11 from which a general purpose processing computer 12 derives the desired arrays.
This approach has distinct advantages of reduced field hardware and associated operational problems, as well as the ability to compensate digitally in the general purpose computer for variations in specific receiver group parameters such as their sensitivities. However, a serious disadvantage is the inefficiency associated with the requirement of a general purpose computer for performing routine array formations better handled by a more specialized system. Furthermore, the resultant array traces are not readily available for field observation unless a computer is available for that purpose.
With reference to FIG. 1C, the second general approach may be seen depicted therein. Here, the desired arrays to be formed are created in the field, by use of analog methods. Thus, a precision resistor network is provided for supplying desired weighting coefficients for each of the component receiver signals as well as an operational amplifier for summing together various of the weighted samples comprising an array. Similarly, if another array is desired, another resistor network 15 and associated summing amplifier 16 would be required. For arrays comprised of large numbers of groups it may even be necessary to sum partial sums from amplifiers 14 and 16 into an additional amplifier 18.
Again, this approach has obvious benefits, most important of which is the elimination of the need for the general purpose processing computers of FIGS. 1A and 1B to perform the routine array forming. However, several serious drawbacks are nevertheless also associated with this technique. First, there are extremely difficult problems to overcome associated with the analog circuitry itself. These include the tremendous increase in hardware cost, size, maintenance, and difficulty of fabrication due to the hundreds of required patch connections, highly accurate and temperature-stable operational amplifiers and precision resistors, and the like. Problems are also experienced in degradation of signal to noise ratios due to summing amplifier drift and noise as well as noise and reliability problems associated with the various patchboards, connectors, and related wiring required in analog systems.
Yet another serious problem with the analog approach is the relative inflexiblity of such systems due to preselected weighting resistor values, limited patchboard connections and amplifier inputs, and the like. Moreover, reconfiguration of the arrays, system testing, and problem isolation was often manual and thus slow and unreliable.
Even with attempts to alleviate some of the aforementioned problems by providing for a more automated analog system with voltage controlled switches and variable gain amplifiers, severe problems still remained. Not the least of these was the difficulty of a host computer interacting and communicating with the array forming circuitry, such as in the performance of automated testing, and reconfiguring functions, again due to the analog nature of the system.